In order to improve the quality of products, in particular series manufactured products with hollow spaces (vessels, housings, heat exchangers, condensers, etc.), the deployment of helium leak detection methods is becoming more and more popular so as to determine and sort out faulty products at an early stage. For reasons of time and cost, the cycle time (time required to test each test body) should be as short as possible.
In a vacuum leak detection process, a light gas, especially helium, is employed as the test gas. In the case of the presence of a leak, this enters into the evacuated test body and it is recorded with the aid of a mass spectrometer. The test body is evacuated with the aid of a fore-vacuum pump accommodated in the leak detector. A high-vacuum pump serves to evacuate the mass spectrometer, since the operating pressure of a mass spectrometer must not exceed a maximum of 10.sup.-4 mbar. One test body at a time is evacuated, and after the pressure (P.sub.u)(switchover pressure) has been attained, the instrument switches over from the evacuation mode to the measurement mode. P.sub.u is the pressure that prevails at the intermediate inlet of the high-vacuum pump. It must be selected to be adequately low, so that the pressure in the mass spectrometer will not exceed the maximum admissible operating pressure.